1. Field of the Invention
The present invention relates to a magnetoresistance effect film for reading the magnetic field intensity of a magnetic recording medium or the like as a signal and, in particular, to a magnetoresistance effect film which is capable of reading a small magnetic field change as a greater electrical resistance change signal, and further relates to a magnetoresistance effect type head using such a magnetoresistance effect film. They are mainly incorporated in, for example, hard disk drives so as to be used.
2. Description of the Prior Art
Recently, following the high densification of hard disks, highly-sensitive heads with high outputs have been demanded. In response to these demands, spin valve heads have been developed.
The spin valve head has a structure wherein two ferromagnetic layers are formed via a non-magnetic metal layer, and an antiferromagnetic layer is disposed so as to abut one of the ferromagnetic layers. The ferromagnetic layer abutting the antiferromagnetic layer is in exchange coupling to the antiferromagnetic layer so that the magnetization of the ferromagnetic layer is fixed (pinned) in one direction. The magnetization of the other ferromagnetic layer is freely rotated following the change of the external magnetic field. In the spin valve, the MR change is realized by a difference in relative angles of spins between the two ferromagnetic layers. Therefore, the exchange coupling between the antiferromagnetic layer and the ferromagnetic layer abutting thereto can be thought as the substance of the spin valve.
As a material of an antiferromagnetic layer used in the spin valve, FeMn, NiMn, PtMn or the like has been well known.
When FeMn is used as the antiferromagnetic layer, the exchange coupling is generated relative to the ferromagnetic layer immediately after the formation of a film. Thus, a heat treatment for generating the exchange coupling is not required after the film formation. However, there is raised a limitation in order of the film formation that the antiferromagnetic layer should be formed after the formation of the ferromagnetic layer. Further, when FeMn is used, there is a problem that a blocking temperature is low, i.e. about 150 to 170.degree. C. The blocking temperature is a temperature at which the exchange coupling pinning a magnetic layer is lost.
On the other hand, when NiMn or PtMn is used as the antiferromagnetic layer, the blocking temperature is high, i.e. no lower than 300.degree. C., and further, there is no limitation in order of the formation of the antiferromagnetic layer and the ferromagnetic layer. However, for generating the exchange coupling between the antiferromagnetic layer and the ferromagnetic layer, a heat treatment is required in the magnetic field after stacking both layers. This is because, for NiMn or PtMn to exhibit the antiferromagnetism, a CuAu-I type regular crystal structure having a face centered tetragonal (FCT) structure needs to be formed. The heat treatment in the magnetic field is normally carried out under a temperature condition of 250 to 350.degree. C. The degree of exchange coupling tends to be increased as the temperature is raised. However, if the heat treatment at high temperatures is applied to the spin valve film, mutual diffusion at boundary film surfaces of the stacked layers forming the spin valve film is caused so that a magnetoresistance change ratio (MR ratio) being an important film characteristic of the spin valve film is lowered. Therefore, when the spin valve head is formed, the sensitivity becomes poor and the output becomes small. Further, since the mutual diffusion at the boundary film surfaces of the stacked layers is generated, it is necessary to set large the thickness of the laminate film forming the spin valve film for ensuring a desired spin valve film characteristic. Thus, a product can not be more compact. Further, since the heat treatment after the formation of the spin valve film is a process at high temperatures and for a long time, improvement has also been demanded from the viewpoint of energy and productivity.